Changes in education policy, the accumulation of research evidence that skilled instruction prevents and ameliorates reading failure, accountability requirements, and a new emphasis on multi-tiered interventions in schools are all causing a growing interest in improving teacher knowledge and skill in reading instruction. Consensus frameworks that explain reading development and individual differences provide an outline for what teachers need to know. The details of that content, however, including the English phonological system, the organization of English orthography, and the language structures that are processed during reading and writing, are challenging for teachers to learn. Recent studies are reviewed that investigate the relationship between teacher knowledge, practice, and student outcomes. The paper argues that teachers must have considerable knowledge of language structure, reading development, and pedagogy to differentiate instruction for diverse learners. Policy mandates for improvement of reading achievement should provide for more effective teacher education, as the knowledge base is not learned casually or easily. Research on how teachers best develop expertise should inform our licensing and professional development programs.